Gravity wave activity during winter above esrange

This study of mountain wave propagation over northern Sweden makes use of data from radiosondes, launched at Esrange (67.6°N, 21.0°E), and from the ESRAD, the Esrange VHF Doppler radar, which have been recorded during winter 1996/97 to 2000/01. As the predominant low-level wind direction with high wind speeds above Esrange is from the north-west, and the upper tropospheric winds are most frequently westerly, one might expect critical level filtering to be relatively unimportant in this region. Despite this, it turned out that radar data as well as the radiosonde data show clear evidence of inhibited wave activity due to critical level filtering. Enhanced wave activity is found mainly in cases where no critical level filtering has occurred. Further, an indication for lower temperatures in the stratosphere during enhanced wave activity has been seen. The observed inter-annual variation in wave activity can be explained by differences in the wave generation due to differences in the mean wind speeds during the winters. The ratio of wave activity in the upper troposphere compared to the wave activity in the lower troposphere is clearly influenced by the amount of critical level filtering.